Press or bulk molding compounds (PMC) are thermosetting plastic resins often mixed with stranded reinforcement, fillers, and other additives into a viscous compound for compression or injection molding. Typical PMC formulas include an uncured polyester resin, chopped glass fibers, a catalyst, release agents, pigments, fillers and thickeners. These are then formulated by combining all the ingredients in an intensive mixing process, forming a material with a clay or dough-like consistency that can be molded directly. Molding of PMC can be accomplished by known compression and injection methods. For example, in compression molding, PMC is weighed and placed into an opened mold, the mold closes and the pressure of closing forces material throughout the mold. The molds are heated (to typically 250–300° F.) and the heat facilitates flow and cures the material. Such formed slabs of cured material can then be ground up into granules. Injection molding of PMC is accomplished by placing PMC into a feeder box or a funnel from which material is stuffed into an injector barrel and pushed by means of turning a screw into a molding chamber. Pressure of injection forces material throughout the mold. The mold is heated (250–300° F.) and the heat cures the material. PMC can advantageously be used to produce low-cost parts with a high level of precision, reproducibility, and allow for efficient production of high volumes. Examples of parts made with PMC granules include solid surfaces and cultured marble, which can be used as table and counter tops, covers, wall surfaces, etc.
However, with known PMC compounds, a thickener must be added to the uncured resin to achieve a desired specific gravity. Specific gravity of PMC materials must be controlled to ensure proper manufacture of parts. Known thickeners in use today include magnesium oxide.
It would be desirable to press molding compound which did not require the use of thickeners such as magnesium oxide.